The present invention relates to a tandem-type color image forming apparatus which forms a color image by causing a transferring belt to pass through image forming stations for respective colors that are disposed along the transferring belt. In each image forming station, a charging device, an image writing device, and a developing device are disposed around an image carrier.
For example, Japanese Patent Publication No. 11-84798A discloses a tandem-type color image forming apparatus in which a plurality of image forming stations are disposed along the circulating direction of a transferring belt (i.e., sheet transporting belt) and the image carrier of each image forming station is charged uniformly by a brush roller. In contrast to the corona charging which is another typical charging method, the amount of ozone generated by the brush roller charging is very small. The brush roller charging is particularly advantageous in tandem-type color image forming apparatus because each image forming station has a charging device. Although the above publication discloses no brush roller driving method, a method in which driving force is transmitted from an image carrier to a brush roller in a monochrome image forming apparatus is known (e.g., Japanese Patent Publication No. 2000-29278A). Japanese Patent Publication No. 10-282855A discloses a technique in which toner left on an image carrier after the transfer operation is gathered by a brush roller for charging the image carrier uniformly and the toner thus gathered is returned to the image carrier and collected by a developing device while the image forming is not performed.
Recent image forming apparatus have come to employ jumping development in which development is performed in a state that the developing device is not in contact with the image carrier and an image forming method in which the efficiency of transfer from the image carrier to a recording medium (i.e., a sheet or an intermediate transferring belt) is increased by using toner that is manufactured by a polymerization method and is high in sphericity and uniform in particle diameter and a cleaner (rubber blade or the like) for the image carrier is thereby omitted. As a result, the driving load of the image carrier is reduced in those image forming apparatus. In tandem-type image forming apparatus, it has become possible to drive the respective image carriers with a single motor and hence to reduce the size of the entire apparatus.
However, in tandem-type image forming apparatus, the above-mentioned reduction in the driving load of each image carrier causes a problem that periodic density unevenness or color misregistration may occur due to, for example, the synergistic effect of a backlash in the gears of an image carrier driving system and contact/separation operations of a transferring roller and a cleaning blade to/from the transferring belt (i.e., intermediate transferring belt or sheet transporting belt).
Further, in tandem-type image forming apparatus, to reduce the size of each image forming station, it is necessary to make the outer diameter of the brush roller smaller than that of the image carrier. In a configuration in which the image carriers of the respective image forming stations are driven by a single motor as mentioned above and the brush rollers are driven by the same motor as disclosed in Japanese Patent Publication No. 2000-29278A, to rotate the brush rollers at a higher circumferential speed than the image carriers it is necessary to rotate the smaller-diameter brush rollers at a higher rotation speed than the image carriers, which increases the driving load of the motor.
Japanese Patent Publication No. 62-299873A discloses a configuration in which the contact between the sheet transporting belt and the image carrier of each image forming station is maintained even during a monochrome printing operation and image carriers corresponding to colors for which image formation is not necessary are also rotated.
However, stopping the brush rollers for uniformly charging non-image-formation image carriers during formation of a monochrome image causes the following problems:
(1) The hair ends of the brush are much inclined by the rotation of the image carrier at the portion, in contact with the image carrier, of each brush roller being stopped, as a result of which charging unevenness may occur in the image carrier at intervals corresponding to the rotation period of the brush roller in the next image forming operation.
(2) When monochrome images are printed successively, the hair ends of the brush are worn at the portion, in contact with the image carrier, of the brush roller of each of the image forming stations having color toners, as a result of which charging unevenness may occur in the image carrier at intervals corresponding to the rotation period of the brush roller in the next image forming operation.
On the other hand, Japanese Patent Publication No. 3-288173A discloses a technique in which when a monochrome printing operation is performed in a tandem-type image forming apparatus, the sheet transporting belt is separated from the image carriers of the non-image-formation image forming stations having color toners and those image carriers are stopped, to thereby prevent wear of those image carriers due to contact of recording media or sliding contact of a cleaning blade.
In this case, the following problems occur if the brush rollers for uniformly charging respective non-image-formation image carriers are rotated during a monochrome image forming operation:
(1) The rotating brush roller contacts the image carrier being stopped of each of the image forming stations having color toners and wears only the contact portion of the image carrier layer of the image carrier body. When a color image is formed later, image unevenness may occur in the color image at intervals corresponding to the rotation period of the image carriers, which makes it necessary to replace the image carriers before a prescribed number of color images have been printed.
(2) The rotating brush roller contacts the image carrier being stopped of each of the image forming stations having color toners and hence is worn earlier. The brush roller thus worn can no longer give the image carrier a desired amount of charge. It becomes necessary to replace the brush rollers before a prescribed number of color images have been printed.